1. Field of the Invention
The present invention relates to a charged particle beam apparatus, a charged particle beam focusing method, a microstructure measuring method, a microstructure inspecting method, a semiconductor device manufacturing method, and a program. The present invention is directed to, for example, an inspection, observation, etc. of a microstructure such as a semiconductor device, a photomask or liquid crystal device by use of a charged particle beam.
2. Related Background Art
Various charged particle beam apparatuses are used to observe and evaluate a minute pattern of a semiconductor device, and one representative device is a scanning electron microscope (SEM). The scanning electron microscope is used for shape observation such as the observation of the section of a device pattern, and an apparatus with a measurement function to measure the dimensions, etc. of the device pattern during a manufacturing process is particularly called a critical dimension scanning electron microscope (CD-SEM) and widely used as a tool for in-line process evaluation. In the CD-SEM, the accelerating voltage of an electron beam is reduced to 1 kV or less and the amount of a beam current is held down to a pA order to suppress damages to device performance due to electron beam application, thereby enabling the in-line evaluation of the manufacturing process.
However, there are some materials constituting the device which have properties susceptible to the influence of the electron beam application. Typical examples of which include a resist used in photolithography and an insulating film having a low dielectric constant. These materials not only change their qualities due to the electron beam, but also cause shape variations such as a volume shrinkage. Thus, the shape of the device pattern changes in an application area where the electron beam has been applied to carry out a measurement or observation, and there is therefore the possibility that the pattern dimensions and the thickness of the film depart from specified values leading to the deterioration of the device performance. In order to suppress the damages due to the electron beam, the CD-SEM employs an irradiation condition including a lower accelerating voltage and a lower current, and further employs a method which suppresses the dose amount of the electron beam as a result of reconsidering a method of scanning with the electron beam during a measurement or observation. Various measures for reducing the damages have been taken in connection with the method of applying the electron beam for acquiring an image during the measurement or observation.
On the other hand, extremely high accuracy in the measurement of the pattern dimensions has been requested due to the recent miniaturization of device patterns, and higher accuracy of automatic focusing has been required together with a higher resolution of the electron beam, a higher S/N ratio of an image, an improved algorithm for the measurement of pattern dimensions, etc. Several methods can be given as the examples of the methods of focusing the electron beam. For example, there is a method comprising: changing stepwise the convergence conditions of the electron beam every time the same line is scanned; calculating an integration value of an absolute value of the difference or differential of secondary electron signals in each convergence condition; and determining a convergence condition under which the integration value is maximized as a convergence condition for providing an optimum focal position. Further, there is a method comprising: performing a horizontal scan and a vertical scan instead of a line scan to acquire a two-dimensional image corresponding to each convergence condition; and calculating an integration value of an absolute value of the difference or differential of secondary electron signals between pixels in an image to produce a convergence condition for providing an optimum focal position. Still further, there is a method comprising: acquiring an SEM image with a low magnifying power in advance; and scanning it with an electron beam perpendicularly to the edge of the SEM image to search for a focal position (e.g., Japanese Patent Publication Laid-open (kokai) No. 5 (1993)-190132).
However, all the focusing methods mentioned above repeatedly scan the same place while changing the convergence condition of the electron beam, and therefore have a disadvantage that the influence of focusing operation on the device pattern is greater in the case of a material damaged by the application of the electron beam. Actually, as compared with the irradiation amount for acquiring an image for use in, for example, a dimensional measurement, the amount of electrons applied to a specimen tends to be greater in the case of focusing. This is attributed to, for example, processing with a high magnification for higher accuracy of focusing, longer processing time due to more finely cut variations of the convergence condition for accurately calculating the maximum value of the integration value, and a greater number of times of repeated scans per convergence condition for improving the S/N ratio of an image. In addition, information on the edge of a pattern alone is generally used for focusing, but the methods mentioned above cause the damage from irradiation even to places which are not necessary for focusing because the whole area is irradiated.
One method of minimizing the beam application to a specimen is, for example, a technique disclosed in Japanese Patent Publication Laid-open No. 7-161327. The technique in Japanese Patent Publication Laid-open No. 7-161327 provides a method in which the convergence condition is changed synchronously with the vertical scan when a two-dimensional image is acquired. According to this method, the convergence condition is set by a beam scan for one screen, and the maximum of the integration value is calculated from a resulting image, such that an optimum convergence condition can be calculated. However, an offset is provided in the convergence condition to carry out repeated scans when there are a small number of edges of patterns, so that the same place is repeatedly scanned, and the damage to the pattern is increased.